From: Michal Hocko <mhocko@kernel.org>
To: Wei Yang <richard.weiyang@gmail.com>
Cc: akpm@linux-foundation.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: use BITS_PER_LONG to unify the definition in page->flags
Date: Sun, 19 Mar 2017 11:08:22 -0400 [thread overview]
Message-ID: <20170319150822.GC12414@dhcp22.suse.cz> (raw)
In-Reply-To: <20170319150345.GA34657@WeideMacBook-Pro.local>
On Sun 19-03-17 23:03:45, Wei Yang wrote:
> On Sun, Mar 19, 2017 at 10:30:13AM -0400, Michal Hocko wrote:
> >On Sat 18-03-17 08:39:14, Wei Yang wrote:
> >> The field page->flags is defined as unsigned long and is divided into
> >> several parts to store different information of the page, like section,
> >> node, zone. Which means all parts must sit in the one "unsigned
> >> long".
> >>
> >> BITS_PER_LONG is used in several places to ensure this applies.
> >>
> >> #if SECTIONS_WIDTH+NODES_WIDTH+ZONES_WIDTH > BITS_PER_LONG - NR_PAGEFLAGS
> >> #if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT <= BITS_PER_LONG - NR_PAGEFLAGS
> >> #if SECTIONS_WIDTH+ZONES_WIDTH+NODES_SHIFT+LAST_CPUPID_SHIFT <= BITS_PER_LONG - NR_PAGEFLAGS
> >>
> >> While we use "sizeof(unsigned long) * 8" in the definition of
> >> SECTIONS_PGOFF
> >>
> >> #define SECTIONS_PGOFF ((sizeof(unsigned long)*8) - SECTIONS_WIDTH)
> >>
> >> This may not be that obvious for audience to catch the point.
> >>
> >> This patch replaces the "sizeof(unsigned long) * 8" with BITS_PER_LONG to
> >> make all this consistent.
> >
> >I am not really sure this is an improvement. page::flags is unsigned
> >long nad the current code reflects that type.
> >
>
> Hi, Michal
>
> Glad to hear from you.
>
> I think the purpose of definition BITS_PER_LONG is more easily to let audience
> know it is the number of bits of type long. If it has no improvement, we don't
> need to define a specific macro .
>
> And as you could see, several related macros use BITS_PER_LONG in their
> definition. After this change, all of them will have a consistent definition.
>
> After this change, code looks more neat :-)
>
> So it looks more reasonable to use this.
I do not think that this is sufficient to justify the change.
--
Michal Hocko
SUSE Labs
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next prev parent reply other threads:[~2017-03-19 15:08 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-18 0:39 Wei Yang
2017-03-19 14:30 ` Michal Hocko
2017-03-19 15:03 ` Wei Yang
2017-03-19 15:08 ` Michal Hocko [this message]
2017-03-19 16:05 ` Wei Yang
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